For example, panel parts for forming various kinds of bodies of cars or the like are press formed parts manufactured by pressing metal plates using press dies. In order to efficiently produce formed parts having a good quality in which cracks, wrinkles or the like are not generated, a simulation of press forming is performed with creating a model of the press die at designing the press formed parts.
Generally, methods for press forming (hereinafter called “pressing method”) are categorized into a forming (bending) and a drawing.
The forming (bending) is a process to press a center portion of a blank material using a die and a punch under the condition such that an outer periphery of the blank material is free. Therefore, it is used for forming a frame part having a hat-type cross-sectional shape or the like. Since this forming (bending) is basically bending process, a compact pressing machine having a rather low load can be used. Consequently, it is easy to save energy and a cost for manufacturing the press die is relatively low. A spring back, however, is easily occurred and it is difficult to retain a shape and secure a dimensional accuracy.
The drawing is a process to press down a blank material together with a die and a blank holder towards a punch under the condition such that an outer periphery of the blank material is held by the die and the blank holder. Therefore, the drawing is used for most of the formation of the deep drawn vessels or the like. The drawing is good at shape freezability, and capable of enhancing strength of formed products. Further, it is possible to restrain generation of wrinkles or cracks on the formed products by controlling a blank holding force. A manufacturing cost of the press die, however, is increased because the blank holder as a die and a drive mechanism thereof are required. Further, since the drawing is a process to use elongation of materials, it is difficult to secure a pressing capability to a high tensile steel plate which recently attracts an attention.
Additionally, in the pressing process, there is a case of press forming under the condition such that a central portion of a blank material is pressed onto a punch by using a spring pad (hereinafter simply called “pad”). In the case of using the pad, a lift or a displacement of the blank material relative to the punch during press forming may be suppressed, which may secure a dimensional accuracy of the formed product. A manufacturing cost of the press die becomes, however, high because the pad and the drive mechanism thereof are required.
Accordingly, it is necessary to apply suitable pressing process with considering the above mentioned features thereof at designing formed products.
Conventionally, in order to manufacture a shape model of the press die for pressing process as mentioned above, an engineer uses CAD data specifying a product shape to design a shape of the formed product configured by a product portion and its peripheral portions which are comprised of an addendum portion and a blank holder portion before trimming, and then designs a die, a punch and the like, which compose a press die so as to follow the shape of the formed product.
At this moment, in order to design a press die which can produce a formed product having a good quality and a high yield rate, specifically to design an addendum surface or a blank holder surface efficiently, knowhow of an experienced engineer for designing a press die is required. If an engineer who does not have such enough knowhow for designing a press die designs a press die, a correction of a shape of the press die must be repeated based on the simulation result, and thereby requiring many works and man hours.
Further, as the above mentioned method, in order to deign a press die shape from scratch by using CAD data specifying a product shape, many operations such as creating an adequate surface and adding a fillet on the CAD are required. Therefore, it takes a lot of time and effort until the engineer becomes familiar with the CAD operation.
Relating to this issue, there is used recently a system to create a press die shape automatically from a product shape with using a computer.
For example, the patent document 1 describes a system for creating an addendum shape which generates an addendum shape of the press die from a product shape automatically. According to the system for creating an addendum shape, an addendum surface may be created by setting a plurality of feature points positioning along the circumference of an outline of the product shape and/or along the circumference of a two or three dimensional punch opening line, placing a plurality of addendum cross-sectional planes which connect the outline of the product shape and the two or three dimensional punch opening line with using the feature points as at least one end point thereof, and setting the shape of the addendum cross-sectional planes to be arranged respectively, and connecting the adjacent addendum cross-sectional planes sequentially.